As is well known, bipolar transistors, in their most elemental form, comprise a sandwich made up of three layers of semiconducting material, the middle layer being of an opposite conductivity type to the outer layers. Much work has been done on optimizing both the dimensions of the three layers as well as determining the best way to distribute dopants within them. For example, in the graded-base transistor, first developed over 40 years ago, it was found that the frequency response of a transistor could be increased by providing a built-in field across the base region to aid the transport of carriers from emitter to collector. In order to produce such field, a resistivity gradient was introduced into the base material as a natural byproduct of the diffusion process.
In FIG. 1a we show a generalized curve of dopant concentration through the crosssection of a transistor. Region 1 of N+ material represents the emitter, region 2 of P type material constitutes the base, and region 3 of N type material forms the collector. Each region was originally formed by using diffusion and/or ion implantation, generally followed by a drive-in diffusion for the purpose of controlling exactly where the interface occurs. In FIG. 1b we show a typical I-V curve 4 (collector current vs. collector to emitter voltage) for a device of this sort. It is evident that, once the knee of the curve has been passed, the current-voltage relationship is linear.
More recently, it has been found that if the base of the device is subdivided into two layers, with the layer closest to the emitter having lower resistivity, a more complicated relationship between I.sub.C and the V.sub.CE results. In FIG. 2a, region 21 of P+ material has been introduced closest to the emitter while region 22 of P type material is the one closest to the collector. This results in the I-V curve seen in FIG. 2b where the curve is seen to be made up of two distinct parts, 25 and 26. Depending on the applied voltage being used, such a device can operate at low gain, with accompanying low power consumption, or it can be operated with higher gain and better current driving capability at higher voltages.
As we will show below, the device and method of the present invention are novel but, nevertheless, there already exists a considerable literature that is relevant to their review and understanding. For example, U.S. Pat. No. 5,213,988 Yamauchi et al.), U.S. Pat. No. 4,866,000 (Okita), U.S. Pat. No. 4,347,654 (Allen et al.), and U.S. Pat. No. 5,130,262 (Masuelier et al.) all describe bipolar transistors having double bases comprised of the P+ and P- regions. In U.S. Pat. No. 5,480,816 Uga et al. show a bipolar transistor having three base layers, each with a different dopant concentration:- a P+ primary base layer and a secondary base layer made up of a P- and a P layer.
In U.S. Pat. No. 5,569,612 Frisina discloses a process for manufacturing a bipolar transistor. An N type region is first formed by epitaxial growth over N+ material following which aluminum ions (boron being counter-indicated) are implanted into the epitaxial layer to form a lightly doped P base. This is followed by the formation of a P+ base within the P base, this time using boron ions.
In U.S. Pat. No. 5,496,746, Matthews shows a bipolar transistor having a base layer comprising a buried P+ layer with an intrinsic base and a P- layer. The method of preparation of this device is quite different from that taught by the present invention.
These prior art devices, while having some similarity to the present invention, differ significantly in their internal dimensions and dopant concentrations and therefore provide different I-V curves, breakdown voltages, etc. from those generated by the present invention. Additionally, the present invention teaches a unique process for the manufacture of the device.